Described below is a dust line for transporting dust, having at least one optical sensor for monitoring a property of the dust. Also described below is a method for measuring a property of dust in a dust line.
Dust lines are used in a number of automated processes for transporting dust, wherein the dust is either transported in a targeted manner to the location of use thereof or else transported away from the location of production thereof. Below, dust is understood to mean an accumulation of solid particles, the particle diameter of which is significantly below 1 mm, usually below 100 μm. Dust swirling in air can levitate for a long time and it can therefore be transported, even in whirled-up form, together with an air flow by way of pneumatically operated dust lines. The most important application of such dust lines is found in coal power plants, in which finely ground coal dust with a particle diameter of usually at most 0.5 mm is fed to a burner by way of a dust line. However, there are also other automated processes in which amounts of dust are transported by way of dust lines, for example when supplying flour, cocoa or starch in food production or when removing wood dusts and metal dusts in material processing.
Monitoring the composition of the transported dusts is desirable for many such processes. For reasons of quality control and of monitoring the processes, a measurement and a monitoring of mean particle dimensions, moisture content and other chemical compositions may be relevant. When supplying coal dust in a coal power plant, the calorific value of the coal in particular is an important parameter that should be monitored. The calorific value is a measure of the energy released during combustion per unit mass of fuel. For coal dust, the calorific value depends, inter alia, on the moisture of the coal dust, on the chemical composition of the coal particles and on the particle dimension of the coal dust. During the operation of a power plant, all these parameters should be kept within a predetermined process window, wherein the predetermined process window may also vary in time, for example if the nominal power of the power plant changes throughout the day.
In principle, monitoring the dust properties by optical measuring processes is desirable. However, it is very difficult to perform optical measurements within a dust line since an optical probe within the line is exposed to high wear-and-tear. Very strong abrasion occurs at the walls of dust lines, particularly when transporting coal dust, and so an optical measurement probe with a sensitive optical window is damaged very quickly. All that is known is an option for a reflection measurement, in which a measurement probe is installed flush with the inner wall of the dust line. However, the wear-and-tear is also great in this case. Cleaning or replacing the probe is very difficult and complicated under normal process conditions. Since, in principle, all combustible dusts such as coal dust, wood dust, flour, cocoa, starch and cellulose dust are explosive, such a dust line may be operated in an explosion-protected manner. This precludes regular cleaning, servicing or the replacement of an optical measurement probe, as well as the use of some other measurement methods, for example electric measurement methods within the dust line.